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I                           124.                                Case-

'                        in Fig.   117 could be stamped by means of the tool shewn in

Pig.   121, the hot iron being placed in the hollow H, and the
:                        hammer brought down upon it,    The ragged portions are after-

wards chipped off the forging. Usually these stamping tools are
made of massive cast iron, but if they are to be used extensively
cast steel will be found necessary. Other examples of work
suitable for stamping are shewn in Fig. 1210, where A is a
spanner, B a double eye, c the centre portion of a screwing stock,
D the handle portion of a lever, and E the boss part of the same
lever. (See Appendix //-,/. 802.)

,                              Before leaving the smithy two processes should be explained,

because they are as a rule performed by the smith.    These are
•;                        the methods of hardening wrought iron and steel.   Cast iron, as

j,ji      "                        we have seen in Chapter 1, can be easily hardened at the surface

ij     •"                        by chilling, this taking place -while the casting is in course of

IVA                               formation.    Wrought iron and steel are hardened after the article

^                                is completed.

V Case-hardening.—This is the name given to the process
by which wrought iron objects are hardened to a depth of from
one-eighth to three-sixteenths of an inch below the surface. After
forging the work is machined and polished, and is then made to
absorb carbon by being placed in air-tight boxes or Ła$es in con-
tact with some substance rich in carbon, being strongly heated
while in that condition. The method is much the same as that
pursued in the cementation process (Fig. 88), and it will therefore
be seen that the iron at the surface is converted into a film or
^                        case of steel, the only difference from the cementation process

being that the heat is merely kept on long enough to case tlie Iron
with steel and not to steel It qtilte through. WMlethe iron is left,
then, hard at the surface the inside remains tough, and is as
•capable as ever of enduring "vibration. The boxes may be either
made of sheet iron, or may be fireclay retorts similar to those in
use at gas works, and provided with a lid to keep them air-tight.
They may be heated as In Kg. 88, and the substance put In
contact with the iron is not wood charcoal, as in cementation, but
animal charcoal in the form of bones; for it is found, why It is
not quite clear, that if nitrogen be present the carbon will unite
more rapidly with the iron, Other substances may he used, such